Prevalent winds as defined herein depend on a specific location, and they move statistically most often in the same direction. For any given period, usually a month, a small percentage of these prevalent winds will contain the bulk of the available energy and are called energy winds.
In the United States prevalent energy winds, having a 10 m.p.h. or more average yearly wind velocity, occur in a north-and-south strip 350 miles wide midway between the Atlantic and Pacific Oceans, on the littoral of the Great Lakes, at the Atlantic Seaboard, the Gulf Coast, and the Pacific Ocean near San Francisco.
The first successful attempt in the United States at producing electric power to feed a utility network was in Vermont and used a two-bladed windmill mounted on a 150 foot tower. The 175 foot blades were pitch controlled and drove an AC alternator at constant speed. The 1,250 KW alternator produced 805 Kwh per KW per year over a 221/2 day period of March 1945. One of the spars subsequently failed, and the project was abandoned.
All wind conversion systems that use pitch control are inefficient and failure prone. Two-bladed systems are classified as high speed windmills and must operate with a tip speed ratio of 5.5 during all wind velocities to provide efficient results. Large high-speed windmills develop high centrifugal forces during energy winds and must be shut down at times to prevent blade failure. The potential for blade damage is one reason for using pitch control.
Pitch control systems can be subject to blade flutter. An air foil is flutter free when small angles of attack are used. Systems using pitch control must withstand high dynamic loads due to flutter and must withstand high centrifugal loads due to the weight of extremely long blades.
A practical method for avoiding failure prone and inefficient systems is to use a multi-bladed turbine with constant pitch. A multi-bladed turbine can use an average angle of approach equal to 45.degree. and an angle of attack under 4% for most wind velocities. Deutsches Reich Pat. No. 371,459 teaches the use of a multi-bladed turbine with constant pitch, but to my knowledge such a system has never been constructed.
There are three scientific principles that can be applied in evaluating wind driven energy systems. The first principle is that, during maximum power removal, the quantity of air passing through a wind energy system is reduced by one-half of the quantity that would flow across the same space if the system was not there. In fact, if the speed of the air is not reduced there is no power removal. Republique Francaise Pat. No. 1,098,995 is an example of a device that violates the first principle, and, of course, is not feasible.
The second principle is that a loss of static pressure occurs as air passes through a wind energy system. The magnitude of this pressure loss is a function of the Reynold's number. At small Reynold's numbers, the loss of static pressure is so high that the air will not flow. With Reynold's numbers of one million (1,000,000) or larger the static pressure losses are less than two percent, and the flow isn't prevented.
Any device mounted on a pole, tower, or small turntable with the exception of a windmill is not feasible because of high static pressure losses. Venturi tubes and shrouds are examples of devices that prevent flow if the device is too small.
The following patents are examples of devices that violate the second principle and are not feasible: U.S. Pat. Nos. (984,599), (1,345,022), (2,563,279), (3,740,565), (3,878,913), and (3,883,750); British Pat. No. 162,999; and Republique Francaise Pat. Nos. (546,417), (989,170), (1,011,132), and (1,098,995).
The third principle is that the change in pressure across a system is equal to, or greater than, the change in pressure through the system. The change in pressure across a system is in reference to the total pressure of the wind at the entry when subtracted by the static pressure of the air behind the system. The change in pressure through a system is in reference to the total pressure of the wind at the entry subtracted by the total pressure at the exhaust.
The pressure drop across a system can be maintained by controlling the slipstream of the wind moving around the system. The further a slipstream is moved away from a power plant, the greater the pressure drop. The pressure drop across a device is a function of the device's size and shape. A surface of a frustum of a right circular cone will give a higher pressure drop than a surface of a thick flat circular ring, and a surface of a frustum of a right circular cone fouled by one or more thick flat circular rings would provide a higher pressure drop than a surface of a frustum of a right circular cone for any given inner and outer radius.
The shape of a slipstream in front of a windmill is like a surface of a segment of a perboloid of revolution. When a windmill is placed inside the walls of a device, the walls must not interfere with the slipstream of the windmill. Interference will cause static pressure losses, and static pressure losses will cause the air to stagnate in front of the device and lead to system inefficiency.
When a windmill is placed in the entry of a venturi tube, the walls of the venturi tube interfere with the natural slipstream behind the windmill and cause static pressure loss.
British Pat. No. 162,999 and Republique Francaise Pat. Nos. 735,040 and 989,170 are examples of devices that will stagnate the wind because the wind cannot expand behind the blades.
When the slipstream of one device interferes with the slipstream of a second device, a static pressure loss will result. When a windmill is placed within the throat of a venturi tube, the natural slipstream in front of the windmill will interfere with the converging slipstream of the collector and cause static pressure loss. U.S. Pat. Nos. 984,599, 1,345,022, and 3,883,750; British Pat. No. 162,999; and Republique Francaise Pat. Nos. 546,417, 989,170, and 1,011,132 are examples of devices that will stagnate the wind, and the wind is accordingly prevented from accelerating.
The demand for electric power is increasing at an increasing rate, but the availability of domestic oil and natural gas for conversion to electric power is decreasing at an increasing rate. The number of coal fired and nuclear power plants being constructed annually is increasing at the maximum possible increasing rate, but it is not possible to keep up with the demand.
The United States must have new energy sources that can replace its oil and natural gas imports and help meet its demand. The wind is one of the few energy sources that is not being used effectively. Its availability is nearly unlimited and it is everlasting. What is needed is a wind driven power plant that can efficiently convert wind energy into electric power.